This invention relates to inverter devices, and more particularly to driver circuits for driving inverter devices wherein charge-pumping type gate driver circuits drive the gates of the semiconductor switching elements (such as IGBTs (Insulated Gate Bipolar Transistor) or MOSFETs) of the inverter circuit.
FIG. 6 is a circuit diagram showing a conventional half-bridge inverter driver circuit. The half-bridge inverter driver circuit of FIG. 6 may be also utilized in combination for driving the single-phase and three-phase inverter circuits shown in FIGS. 8 and 9. FIG. 7 is a circuit diagram showing the circuit organization of the lower arm driver voltage source. The DC power supplied from a DC power source 1 is converted into an AC power by means of the circuit of FIG. 6. Namely, in response to an upper arm ON/OFF signal 6 and a lower arm ON/OFF signal 7, the upper arm driver circuit 4 and the lower arm driver circuit 5 alternately turns on and off the upper arm IGBT 2 and the lower arm IGBT 3, respectively, thereby alternately supplying at the inverter output terminal the positive and the negative voltage of the DC power source 1.
Thus, the lower arm driver circuit 5, supplied from a lower arm driver voltage source 8, turns on and off the lower arm IGBT 3 in response to the lower arm ON/OFF signal 7. More specifically, when the lower arm ON/OFF signal 7 is an ON command, the lower arm driver circuit 5 applies the output voltage of the lower arm driver voltage source 8 on the gate G of the lower arm IGBT 3, thereby turning on the lower arm IGBT 3. On the other hand, when the lower arm ON/OFF signal 7 is an OFF command, the lower arm driver circuit 5 short-circuits between the gate G and the emitter E of the lower arm IGBT 3, to turn off the lower arm IGBT 3.
The ON/OFF of the upper arm IGBT 2 is effected substantially in a similar manner. However, the upper arm driver circuit 4 is supplied from a capacitor 10. Namely, during the time when the lower arm IGBT 3 is turned on, the capacitor 10 is charged by the lower arm driver voltage source 8 via a diode 9 and the lower arm IGBT 3, as indicated by dotted arrows in FIG. 6. The resulting voltage across the capacitor 10 is supplied to the upper arm driver circuit 4 to be applied on the gate G of the upper arm IGBT 2 in response to the upper arm ON/OFF signal 6 when the upper arm IGBT 2 is to be turned on.
The upper arm IGBT 2 and the lower arm IGBT 3 are turned on and off complementarily: they are never turned on simultaneously. Thus, when the lower arm IGBT 3 is turned on, the upper arm IGBT 2 is turned off. During the intervals when the upper arm IGBT 2 is turned off and the lower arm IGBT 3 is turned on, the capacitor 10 is charged by the lower arm driver voltage source 8. When the upper arm ON/OFF signal 6 is an ON command, the upper arm driver circuit 4 applies the voltage of the capacitor 10 across the gate G and the emitter E of the upper arm IGBT 2, thereby turning on the upper arm IGBT 2.
As described above, when the lower arm IGBT 3 is turned on, the output voltage of the lower arm driver voltage source 8 is pumped up to the capacitor 10 via the diode 9 and the lower arm IGBT 3, the capacitor 10 thereby serving as a driver circuit voltage source for the upper arm IGBT 2. This driver circuit system is referred to as the charge-pumping type.
The lower arm driver voltage source 8 is organized as shown in FIG. 7. The current from the DC power source 1 is subjected to a high frequency PWM switching via a power MOSFET 101, which is turned on and off by a control IC 102 for controlling the output voltage V.sub.CC. Thus, the voltage from the DC power source 1 undergoes DC/DC conversion via a transformer 100, the output of which is rectified via the diode 104 and smoothed via a capacitor 105. The resulting output voltage V.sub.CC, obtained across the capacitor 105, is supplied to the lower arm driver circuit 5. The control IC 102 controls the output voltage V.sub.CC to a predetermined level irrespective of the voltage or output current variations of the DC power source 1. Further, the control IC 102 detects the voltage across the current detector resistor 103 and thereby determines the current flowing through the power MOSFET 101, to suppress an over-current. The control IC 102 thus prevents occurrences of failure of the power MOSFET 101 resulting from an over-current that may flow through the power MOSFET 101 upon building-up of the source voltage 8. Further, by detecting the load current of the lower arm driver voltage source 8 indirectly, the control IC 102 prevents occurrences of failure of the voltage source circuit or a shortening of the life thereof due to an inordinate increase in the load current.
The above conventional driver circuit for an inverter device, however, has the following disadvantage.
When the amount of charge stored in the capacitor 10 is null or small, as when the voltage source 8 is being built-up, the lower arm IGBT 3 is turned on before the upper arm IGBT 2 begins to be turned on and off, to pump up the change from the lower arm driver voltage source 8 to the capacitor 10. Since, however, the load of the lower arm driver voltage source 8 is capacitive during this initial charge period of the capacitor 10, a large current flows through the load to disturb the voltage control system. Thus, the output voltage may become unstable, or an over-current protection circuit may be activated to reduce the output voltage.